class User {

        /**
         * @OneToMany(targetEntity="AppBundleEntityUserDataAttribute", mappedBy="user", cascade={"persist", "remove"})
         * @var DoctrineCommonCollectionsCollection
         */
        private $customAttributes;
}

class UserDataAttribute
{
        /**
         * @ManyToOne(targetEntity="AppBundleEntityUser", inversedBy="customAttributes")
         * @JoinColumn(name="user_id", referencedColumnName="id")
         */
        private $user;
}

INSERT INTO user_data_attr (user_id) VALUES (null);

INSERT INTO user_data_attr (userId) VALUES (2323 (the actual value)));

INSERT INTO user_data_attr (user_id) VALUES (2323);